redis::client: Add exponential backoff on reconnects

`:reconnect_delay` option can be used alongside `:reconnect_attempts`
option to provide a try->fail->retry mechanism for connecting to
Redis with exponentinal backoff[1].

By using `:reconnect_delay` option we can avoid having busy-loops that
try to reconnect to Redis.

`:reconnect_delay_max` adds a time limit to reconnect retries.

[1]: https://en.wikipedia.org/wiki/Exponential_backoff

Author: avgerin0s

lib/redis/client.rb

@@ -18,6 +18,8 @@ class Client
       :id => nil,
       :tcp_keepalive => 0,
       :reconnect_attempts => 1,
+      :reconnect_delay => 0,
+      :reconnect_delay_max => 0.5,
       :inherit_socket => false
     }
 
@@ -375,6 +377,10 @@ def ensure_connected
         disconnect
 
         if attempts <= @options[:reconnect_attempts] && @reconnect
+          sleep_t = [(@options[:reconnect_delay] * 2**(attempts-1)),
+                     @options[:reconnect_delay_max]].min
+
+          Kernel.sleep(sleep_t)
           retry
         else
           raise
@@ -452,6 +458,8 @@ def _parse_options(options)
       options[:write_timeout]   = Float(options[:write_timeout])
 
       options[:reconnect_attempts] = options[:reconnect_attempts].to_i
+      options[:reconnect_delay] = options[:reconnect_delay].to_f
+      options[:reconnect_delay_max] = options[:reconnect_delay_max].to_f
 
       options[:db] = options[:db].to_i
       options[:driver] = _parse_driver(options[:driver]) || Connection.drivers.last

test/internals_test.rb

@@ -202,6 +202,19 @@ def test_retry_with_custom_reconnect_attempts_can_still_fail
     end
   end
 
+  def test_retry_with_custom_reconnect_attempts_and_exponential_backoff
+    close_on_ping([0, 1, 2], :reconnect_attempts => 3,
+                             :reconnect_delay_max => 0.5,
+                             :reconnect_delay => 0.01) do |redis|
+
+      Kernel.expects(:sleep).with(0.01).returns(true)
+      Kernel.expects(:sleep).with(0.02).returns(true)
+      Kernel.expects(:sleep).with(0.04).returns(true)
+
+      assert_equal "3", redis.ping
+    end
+  end
+
   def test_don_t_retry_when_second_read_in_pipeline_raises_econnreset
     close_on_ping([1]) do |redis|
       assert_raise Redis::ConnectionError do